This commit updates all `err == io.EOF` comparisons to use
`errors.Is(err, io.EOF)`. This is necessary when the error may have been
wrapped and fixes at least one current breakage (in `tsh request ls`).
`golang.org/x/tools/refactor/eg` was very handy for this, I used the
following template:
```go
package teleport
import (
"errors"
"io"
)
func before(err error) bool { return err == io.EOF }
func after(err error) bool { return errors.Is(err, io.EOF) }
```
* Vendor slack plugin and supporting libraries
* Fix up plugin integration tests (wip)
* Run GCI on vendored code
* Use newtype instead of type alias
golangci-lint currently panics on this,
"skip-files" et al don't help, as it is a linter panic, not an error
See https://github.com/dominikh/go-tools/commit/d717045480130461e1e777ead7f2981621f0d9f0
* Remove long-runing plugins tests from difftest
* Move access plugin tests to unit-tests-integrations
Nic Klaassen